Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 50

The degree and rate of hydrolysis, and hence the degree and rate of precipitation, can
be controlled through the addition of modifying or chelating agents. These modifying
or chelating agents include chemicals such as alcohols. The precipitation reaction is con-
trolled by the quantity and reactivity of the modifying group. Factors such as coordina-
tion number, hygroscopicity, and reactivity of the alkoxide determine the amount of the
additive required to stabilize the alkoxide. The addition of such modifying agents can also
increase the time to gelation, and therefore can increase the stability or useful lifetime of a
coating solution (Melpolder and Coltrain 1988).
The mode of water absorption and the amount of water available can often have an effect
on the formation of precipitates. For example, when water is adsorbed from the atmo-
sphere, the hydrolysis is gradual and uniform. On the other hand, if water is added drop-
wise, a heterogeneous reaction takes place, with the resultant hydroxide precipitating out
instantaneously (Zheng et al. 1988).

Drying and Sintering of Gels

Drying is the process whereby excess solvents are removed from the pore network and is
probably the most important process in the production of monoliths via sol-gel synthe-
sis. During the drying and firing (sintering) processes, a significant amount of shrinkage
takes place (Figure 2.7). The amount of shrinkage is proportional to the moisture content
(Anderson and Klein 1987). As stated earlier, cracking can be a problem due to the large
capillary stresses generated when pores are small (i.e., less than 20 nm), and also due to the

20 µm

Dip coating

500 nm

Drying

250 nm

Pyrolysis

Sintering

Good Bad

FIGURE 2.7
Schematic diagram showing the drying and sintering stages of gels.